Tom, On 3/7/06 8:03 PM, "Tom Lane" <[EMAIL PROTECTED]> wrote:
> "Luke Lonergan" <[EMAIL PROTECTED]> writes: >> Two passes is the state-of-the-practice on external disk sorts. > > There is no such thing as a fixed number of passes regardless of > available memory and size of the data. While technically correct, the practice shows that two-passes is the norm for the vast majority of cases since 1986: http://research.microsoft.com/~Gray/papers/TandemTR86.3_FastSort.doc Square root of the sort set is the memory requirement for a two pass sort. 10M for 10GB of sort set, for instance. Given the current resource management limitations we live with, you are correct about multi-pass being necessary, but we should be aware that modern commercial databases allocate enough memory to provide two-pass external sorts. - Luke ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
